1. Field of the Invention
The present invention relates to energetic time delay compositions that are useful in military flares, signals, grenades, and various munitions which require a delay between initiation and detonation/ignition; which compositions are environmentally benign and nontoxic, and which repeatably exhibit the desired delay times/burning rates.
2. Background Art
In general terms, energetic materials produce a self-sustained exothermic chemical reaction upon ignition. Pyrotechnic compositions are widely used for a variety of military and civilian applications including explosive and fragmentation munitions, igniters, flares, smoke/gas generators and fireworks. Typically, pyrotechnics are composed of a fuel or combination thereof (e.g., aluminum, tungsten, silicon, boron, and magnesium) that is mixed with an oxidizer or combination thereof (e.g., perchlorates, chlorates, nitrates, and metal oxides)—which ingredients readily react in exothermic, reduction-oxidation (redox) type reactions to produce the desired pyrotechnic effects. These compositions can be tailored to meet various application requirements, such as burning rate, by changing a variety of control parameters including particle size/morphology, stoichiometry, degree of confinement and consolidation density to name a few. For example, some energetic reactants such as nano-thermites can have a burning rate as high as 1000 m/s (meters per second), while pyrotechnic delay compositions can have burning rates as low as 0.6 mm/s (millimeters per second).
Slow-burning delay compositions are uniformly required to produce a repeatable time interval between energetic stages or elements. Such materials are commonly used in delay fuzes that are widely used by the military in flares, signaling devices, grenades, mortars, and artillery projectiles. Having a reliable timed delay in such devices is critical, to provide a safe time interval after the user initiates the device and before the ignition of the device's primary charge, e.g., to provide the desired flare, signal, smoke, or the detonation/fragmentation—such as of a grenade. The delay interval is a function of the time that it takes the slow-burning delay composition to propagate a set distance. Typically, slow-burning delay compositions are housed in aluminum, zinc alloy, or lead tubes, contained within the flares, signals, and grenades or similar devices. The delay composition is ignited by an electrical or chemical igniter and after the slow-burning composition completes its propagation, along the length of its container, it in-turn ignites an output charge, which in-turn ignites the main charge of the device, i.e., the flare, signal, smoke, or detonation component. Further, considering the heat loss resulting from the metal tube enclosures used for such devices, the energetic delay composition must maintain an adequate reaction temperature to avoid being quenched and extinguished by the heat loss to and through the housing itself.
Many of today's pyrotechnic time delay compositions contain environmentally hazardous and toxic materials including heavy metals, chromates, and perchiorates (e.g., BaCrO4, PbCrO4, and KClO4). Even while these materials are facing increasing regulatory scrutiny, they are still in widespread use in military delay systems, due to their proven reliability and their ability to provide a wide range of combustion velocities, i.e., delays. Commonly used compositions include the tungsten delay (W/BaCrO4/KClO4/diatomaceous earth), the manganese delay (Mn/BaCrO4/PbCrO4), the zirconium-nickel delay (Zr—Ni alloy/BaCrO4/KClO4), and the T-10 delay (B/BaCrO4). One of the most versatile of these pyrotechnic delays is the W/BaCrO4/KClO4/diatomaceous earth system (the tungsten delay), that may be tuned to give combustion velocities ranging from 0.6-150 mm/s. This system has proven viable in the small diameter tube housings typically used for military applications—housings where heat losses can be substantial due to the high thermal conductivity of the metal housing (often die-cast zinc alloy or aluminum), and where the housing may have an internal diameter as small as 5 mm. Due to such high heat loss environments, finding alterative energetic materials that do not fail due to combustion front instabilities (e.g., oscillations, pulsations), or simply due to extinction, has proven challenging. Further, the current W/BaCrO4/KClO4/diatomaceous earth compositions are also “gasless” (producing less than 10 ml of gas per gram of composition) allowing them to be used in sealed/semi-sealed housings with a reduced risk of premature case rupture. Finding an alternative which functions and propagates slowly in a high heat loss environment, while also not producing any significant amount of gas, has proven challenging.
Therefore, there is a need in the art for an environmentally benign and sustainable (i.e., safe) replacement for the current environmentally hazardous and toxic commonly used pyrotechnic delay compositions (e.g. W/BaCrO4/KClO4/diatomaceous earth, Mn/BaCrO4/PbCrO4, Zr—Ni alloy/BaCrO4/KClO4, and B/BaCrO4)—a replacement that will repeatably and fully propagate the entire length of the narrow, metal, tube delay housings typically used in military applications and that will burn without generating any significant gas by-products. Further, any replacement should be “tunable” to burn faster or slower—so as to provide a range of desired delay times.